Vacuum pump

ABSTRACT

A vacuum pump ( 10 ) includes a pump casing ( 56 ) having an inlet port ( 54   a ), an outlet port ( 54   b ), and pump chambers ( 50   a - 50   f ) defined therein, rotational shafts  60  having opposite ends rotatably supported by bearings ( 84   a,    84   b ) on the pump casing ( 56 ) and extending longitudinally in the pump casing ( 56 ), and rotors ( 62   a - 62   f ) housed in the pump chambers ( 50   a - 50   f ) and coupled to the rotational shafts ( 60 ) for rotation in unison with the rotational shafts ( 60 ). The pump casing ( 56 ) has an atmosphere-vented compartment ( 52 ) defined therein which is held in fluid communication with the outlet port ( 54   b ) of the pump casing ( 56 ) and vented to the atmosphere.

TECHNICAL FIELD

The present invention relates to a vacuum pump for use in a process such as a CVD process or an etching process which serves as part of fabrication methods for producing semiconductors, liquid crystals, solar cells, LEDs, etc., and more particularly to a vacuum pump for use in a process wherein a sublimable gas or a corrosive gas tends to flow into the vacuum pump.

BACKGROUND ART

Vacuum pumps, which are connected to a vacuum chamber to discharge a process gas introduced in the vacuum chamber, generally include a pump casing having an inlet port and an outlet port and a pump chamber defined therein, and rotors rotatably housed in the pump casing. When the rotors are rotated about their own axes in the pump chamber, the process gas, which has flowed through the inlet port into the pump chamber, is compressed by the rotors and then discharged out of the pump chamber through the outlet port. The rotors are fixedly mounted on respective rotational shafts extending through the pump casing. Each of the rotational shafts has opposite ends rotatably supported by respective bearings that are disposed in respective bearing compartments on respective sides of the pump casing.

Therefore, the inlet port region of the pump casing of the vacuum pump, which is connected to the vacuum chamber, is kept under a vacuum which is at the same level as the vacuum in the vacuum chamber and the outlet port region of the vacuum casing is kept substantially under the atmospheric pressure as it is vented to the atmosphere. The opposite ends of the rotational shafts are rotatably supported by the respective bearings, and are sealed by contact seals or non-contact seals to protect the bearings against damage by products that are generated by the process gas which has entered the bearings. The non-contact seals are widely used to seal the rotational shafts for protection against contact-induced damage of the rotational shafts.

When a vacuum chamber or the like is evacuated by using a multistage vacuum pump having a plurality of pump chambers in multiple stages, for example, the pressure of a process gas in the vacuum pump increases stepwise as the process gas flows through the successive pump chambers, such as first, second, and third pump chambers, of the vacuum pump. In each of the pump chambers, the process gas is higher in pressure on the outlet side thereof than on the inlet side thereof. Therefore, the pressure of the process gas in the final-stage pump chamber is essentially equal to the atmospheric pressure on the outlet side (discharge port) thereof, and is lower than the atmospheric pressure on the inlet side thereof. If the non-contact seals are used to seal the rotational shafts to prevent undue wear on the rotational shafts, then the pressure of the process gas in the bearing compartment, which is adjacent to the final-state pump chamber and houses bearings therein, is commensurate with the pressure (average pressure) in the final-state pump chamber. For example, if the pressure on the outlet side (discharge port) of the final-stage pump chamber is essentially an atmospheric pressure of 760 Torr and the pressure on the inlet side of the final-stage pump chamber is of 200 Torr which is lower than the atmospheric pressure, then the pressure in the bearing compartment that is adjacent to the final-state pump chamber is of about 480 Torr (=(760+200)/2).

The pressure on the inlet side of the final-stage pump chamber changes due to an influx of the process gas from the vacuum chamber or the like. For example, when the process gas flows from the vacuum chamber into the final-stage pump chamber, the pressure on the inlet side of the final-stage pump chamber rises from 200 Torr to 300 Torr. On the other hand, the pressure on the outlet side of the final-stage pump chamber remains substantially unchanged from the atmospheric pressure because the outlet side is vented to the atmosphere through a pump discharge pipe. When the pressure on the inlet side of the final-stage pump chamber rises from 200 Torr to 300 Torr, the pressure (average pressure) in the final-stage pump chamber increases to 530 (=(760+300)/2) Torr, which is higher than the pressure of 480 Torr in the bearing compartment adjacent to the final-state pump chamber.

When the average pressure in the final-stage pump chamber thus becomes higher than the pressure in the bearing compartment adjacent to the final-state pump chamber, the process gas introduced into the final-state pump chamber tends to leak into the bearing compartment. If the process gas contains a sublimable gas or the like, then since the bearing compartment is generally held at a low temperature, products generated by the process gas are deposited on the bearings disposed in the bearing compartment and a lubricant that is used to lubricate the bearings, thereby tending to damage to the bearings.

There has been proposed a dry pump having a hollow thermally insulative intermediate chamber and a cooling passage for passing a refrigerant therethrough, between a pump chamber that is kept at a relatively high temperature and a lubricant chamber that is kept at a relatively low temperature, in order to minimize vaporization of the lubricant for thereby effectively keeping the lubricant in the lubricant chamber at a low temperature while at the same time keeping the pump chamber at a high temperature to lend itself to the discharging of a gas such as a condensable gas or a sublimable gas (see Japanese laid-open patent publication No. 2005-105829).

CITATION LIST Patent Literature

-   [PTL 1] -   Japanese laid-open patent publication No. 2005-105829

SUMMARY OF INVENTION Technical Problem

The dry pump disclosed in Japanese laid-open patent publication No. 2005-105829 serves to minimize vaporization of the lubricant for effectively keeping the lubricant in the lubricant chamber at a low temperature while at the same time keeping the pump chamber at a high temperature to lend itself to the discharging of a gas such as a condensable gas or a sublimable gas. However, this disclosed dry pump has nothing to do with the protection against the process gas of the bearings that is disposed in a bearing compartment disposed on a side of the pump casing. It has widely been customary to introduce a purge gas, such as an N₂ gas or the like, into the non-contact seals on the rotational shafts to prevent the process gas from leaking into the bearings. However, there is a certain limitation on the amount of the purge gas that can be introduced into the non-contact seals on the rotational shafts because the pressure in the pump chamber is adversely affected by an increase in the amount of the purge gas introduced into the non-contact seals on the rotational shafts.

The present invention has been made in view of the above situation in the background art. It is therefore an object of the present invention to provide a vacuum pump which is capable of preventing a process gas introduced into pump chambers from leaking into bearings effectively for thereby protecting the bearings against the process gas.

In order to achieve the above object, the present invention provides a vacuum pump comprising a pump casing having an inlet port, an outlet port, and a pump chamber defined therein, a rotational shaft having opposite ends rotatably supported by bearings and extending longitudinally in the pump casing, and a rotor housed in the pump chamber and coupled to the rotational shaft for rotation in unison with the rotational shaft. The pump casing has an atmosphere-vented compartment defined therein which is held in fluid communication with the outlet port of the pump casing and vented to the atmosphere.

Since the atmosphere-vented compartment which is held in fluid communication with the outlet port of the pump casing and vented to the atmosphere is provided near the outlet port, even when the pressure in the pump chamber changes and non-contact seals are used to seal the rotational shaft, a chamber that is positioned outside the atmosphere-vented compartment and houses one of the bearing therein is maintained substantially at the atmospheric pressure at all times. Therefore, a process gas, which is introduced from a vacuum chamber into the pump chamber, is reliably prevented from leaking into the chamber positioned outside the atmosphere-vented compartment and housing one of the bearings therein. The bearing is thus protected against the process gas.

In a preferred aspect of the present invention, the pump chamber comprises a plurality of pump chambers held in fluid communication with each other, and the rotor comprises a plurality of rotors disposed respectively in the pump chambers for rotation in unison with the rotational shaft.

If the vacuum pump is a multistage vacuum pump, when the outlet side (discharge port) of a final-stage pump chamber is substantially at the atmospheric pressure, the pressure on an inlet side of the final-stage pump chamber is lower than the atmospheric pressure. When the process gas flows from the vacuum chamber into the final-stage pump chamber, the pressure (average pressure) in the final-stage pump chamber changes, e.g., increases. However, since the atmosphere-vented compartment is disposed outside the final-stage pump chamber, such a change in the pressure (average pressure) in the final-stage pump chamber is prevented from affecting the pressure in the chamber positioned outside the atmosphere-vented compartment and housing one of the bearings therein.

In a preferred aspect of the present invention, the vacuum pump further comprises a side panel disposed on an end wall of the pump casing, the rotational shaft extending through the side panel, the side panel having a purge gas passage defined therein for supplying a purge gas to a portion of the rotational shaft which is disposed in the side panel.

Since the purge gas passage is defined in the side panel through which the rotational shaft extends, for supplying the purge gas to the portion of the rotational shaft which is disposed in the side panel, a non-contact seal is provided between the side panel and the rotational shaft for preventing the rotational shaft from suffering contact-induced wear.

According to the present invention, even when the pressure in the pump chamber changes, since the chamber positioned outside the atmosphere-vented compartment and housing one of the bearings therein is kept at the atmospheric pressure at all times, the process gas, which has flowed into the pump chamber, is prevented from leaking into the chamber positioned outside the atmosphere-vented compartment and housing one of the bearings therein. The bearing is thus reliably protected against the process gas.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a vertical sectional front view of a vacuum pump according to an embodiment of the present invention.

FIG. 2 is a vertical sectional side view of a first-stage pump chamber of a main pump of the vacuum pump provided in the vacuum pump shown in FIG. 1.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the present invention will now be described with reference to the drawings. FIG. 1 is a vertical sectional front view of a vacuum pump 10 according to an embodiment of the present invention. As shown in FIG. 1, the vacuum pump 10 includes a booster pump 12 disposed on a vacuum side and a main pump 14 disposed on an atmosphere side, which are connected to each other by a joint pipe 16. In this embodiment, the main pump 14 comprises a six-stage roots vacuum pump, and the booster pump 12 comprises a single-stage roots vacuum pump.

The booster pump 12 includes a pump casing 22 having a substantially cylindrical outer barrel 20 with a pump chamber 18 defined therein, and a pair of rotational shafts 26 disposed through the pump casing 22 and synchronously rotatable about their own axes in respective opposite directions by energizing an electric motor 24. The pump chamber 18 houses a pair of rotor 28, such as two-lobed rotors, rotatably therein with a predetermined clearance therebetween. The rotors 28 are fixedly mounted respectively on the rotational shafts 26. The outer barrel 20 has an inlet port 20 a defined in its wall and connected to a discharge pipe (not shown) extending from a vacuum chamber or the like that is to be evacuated by the vacuum pump 10, and an outlet port 20 b defined in its wall and connected to the joint pipe 16. When the rotors 28 are synchronously rotated about their own axes in the respective opposite directions by energizing the electric motor 24, a process gas from the vacuum chamber or the like flows through the inlet port 24 a into the pump chamber 18, is compressed by the rotors 28 in the pump chamber 18, and is then discharged through the outlet port 20 b into the joint pipe 16. In FIG. 1, only one of the rotational shafts 26, one of the rotors 28, and one of mechanisms for actuating the rotational shafts 26 based on the drive power from the electric motor 24 are illustrated. The other rotational shaft, the other rotor, and the other mechanism are positioned behind away from the viewer of FIG. 1.

In this embodiment, the outer circumferential surface of the outer barrel 20 of the pump casing 22, except the inlet port 20 a and the outlet port 20 b, is surrounded by a heater jacket 30 which is of a substantially hollow cylindrical shape. When energized, the heater jacket 30 heats the interior of the pump chamber 18.

The booster pump 12 is generally kept at a high vacuum level (low pressure level) therein, and has a low temperature as it does not produce much heat of compression. Therefore, even if the pressure in the pump chamber 18 is low, a sublimable substance or the like contained in the process gas, which has flowed into the pump chamber 18, is liable to be deposited on the inner circumferential surface of the pump chamber 18. However, by increasing the temperature in the pump chamber 18 by the heater jacket 30 as described above, the sublimable substance or the like contained in the process gas, which has flowed into the pump chamber 18, is prevented from being deposited on the inner circumferential surface of the pump chamber 18.

Two side panels 32 a, 32 b are disposed respectively on the axial ends of the pump casing 22. The rotational shafts 26 are rotatably supported at their outer ends by bearings 36 a, 36 b housed in bearing housings 34 a, 34 b that are mounted respectively on the side panels 32 a, 32 b. Two lubricant housings 40 a, 40 b for holding a lubricant therein are disposed on respective outer side surfaces of the side panels 32 a, 32 b. The electric motor 24 has a motor housing coupled to one of the lubricant housings 40 b.

The side panels 32 a, 32 b have respective purge gas passages 42 a, 42 b for supplying a purge gas, such as an N₂ gas or the like, to the portions of the rotational shafts 26 in the side panels 32 a, 32 b to prevent the process gas from flowing out of the pump chamber 18 into the bearings 36 a, 36 b. The purge gas supplied from the purge gas passages 42 a, 42 b provides non-contact seals between the side panels 32 a, 32 b and the rotational shafts 26 for protecting the rotational shafts 26 against contact-induced wear.

The main pump 14 of this embodiment comprises a six-stage roots vacuum pump, and includes a pump casing 56 having a substantially cylindrical outer barrel 54 with six pump chambers 50 a-50 f, i.e., first- through sixth-stage pump chambers 50 a-50 f, defined therein, an atmosphere-vented compartment 52 defined therein adjacent to the sixth-stage pump chamber 50 f, and a pair of rotational shafts 60 disposed through the pump casing 56 and synchronously rotatable about their own axes in respective opposite directions by energizing an electric motor 58. The first-stage pump chamber 50 a, which is disposed on a suction side of the main pump 14, houses a pair of rotors 62 a, such as three-lobed rotors, rotatably therein, as shown in FIG. 2. Similarly, the second-stage pump chamber 50 b houses a pair of rotors 62 b, such as three-lobed rotors, rotatably therein, and the third-stage pump chamber 50 c houses a pair of rotors 62 c, such as three-lobed rotors, rotatably therein. The fourth-stage pump chamber 50 d houses a pair of rotors 62 d, such as three-lobed rotors, rotatably therein, the fifth-stage pump chamber 50 e houses a pair of rotors 62 e, such as three-lobed rotors, rotatably therein, and the sixth-stage pump chamber 50 e, which is disposed on a discharge side of the main pump 14, houses a pair of rotors 62 f, such as three-lobed rotors, rotatably therein. One linear array of rotors 62 a-62 f is fixedly mounted on one of the rotational shafts 60, whereas the other linear array of rotors 62 a-62 f is fixedly mounted on the other rotational shaft 60.

The pump casing 56 has a pair of end walls 64 a, 64 b closing the respective opposite ends of the outer barrel 54 and five, i.e., first through fifth, partition walls 66 a-66 e that partition the interior of the outer barrel 54. The end wall 64 a and the first partition wall 66 a define the first-stage pump chamber 50 a therebetween in the outer barrel 54. The first partition wall 66 a and the second partition wall 66 b define the second-stage pump chamber 50 b therebetween in the outer barrel 54. The second partition wall 66 b and the third partition wall 66 c define the third-stage pump chamber 50 c therebetween in the outer barrel 54. The third partition wall 66 c and the fourth partition wall 66 d define the fourth-stage pump chamber 50 d therebetween in the outer barrel 54. The fourth partition wall 66 d and the fifth partition wall 66 e define the fifth-stage pump chamber 50 e therebetween in the outer barrel 54. The fifth partition wall 66 e and the end wall 64 b define the sixth-stage pump chamber 50 f therebetween in the outer barrel 54. The end wall 64 b and a side panel 80 b, which is disposed adjacent and axially spaced from the end wall 64 b, define the atmosphere-vented compartment 52 therebetween.

When the rotors 62 a in the first-stage pump chamber 50 a are synchronously rotated about their own axes in the respective opposite directions by the electric motor 56, a process gas is introduced into the first-stage pump chamber 50 a from an upper inlet side thereof which is connected to the joint pipe 16, is compressed by the rotors 62 a in the first-stage pump chamber 50 a, and is then discharged from the first-stage pump chamber 50 a out of a lower outlet side thereof, as shown in FIG. 2. The process gas is subsequently compressed similarly in the second- through sixth-stage pump chambers 50 b-50 f.

The outer barrel 54 has an inlet port 54 a defined in its side wall which is connected to the joint pipe 16 and held in fluid communication with the upper inlet side of the first-stage pump chamber 50 a, and an outlet port 54 b defined in its side wall which is held in fluid communication with the lower outlet side of the sixth-stage (final-stage) pump chamber 50 f. The outlet port 54 b is also held in fluid communication with the atmosphere-vented compartment 52 through the end wall 64 b. With this structure, the atmosphere-vented compartment 52 is allowed to vent to the atmosphere through the outlet port 54 b. The outer barrel 54 of the pump casing 56 is of a double-walled structure including an inner wall 68 and an outer wall 70 disposed outside of and spaced a certain distance from the inner wall 68, with first through fifth gas passages 72 a-72 e being defined therebetween. Specifically, the first gas passage 72 a extends around the first-stage pump chamber 50 a, and the second gas passage 72 b extends around the second-stage pump chamber 50 b. The third gas passage 72 c extends around the third-stage pump chamber 50 c, the fourth gas passage 72 d extends around the fourth-stage pump chamber 50 d, and the fifth gas passage 72 e extends around the fifth-stage pump chamber 50 e. The fifth gas passage 70 e also extends around the sixth-stage pump chamber 50 f.

The gas passages 72 a-72 e have respective portions held in fluid communication with the respective pump chambers 50 a-50 e through the respective lower outlet sides thereof, and also have respective portions held in fluid communication with the respective pump chambers 50 b-50 f through the respective upper inlet sides thereof. Therefore, as shown in FIG. 2, the process gas that has flowed from the inlet port 54 a into the first-stage pump chamber 50 a through its upper inlet side is compressed in the first-stage pump chamber 50 a, and then flows from the first-stage pump chamber 50 a through its lower outlet side into the first gas passage 72 a. Then, the process gas flows upwardly in the first gas passage 72 a and reaches the upper inlet side of the second-stage pump chamber 50 b. The process gas flows into the second-stage pump chamber 50 b through its upper inlet side and is compressed in the second-stage pump chamber 50 b, and then flows from the second-stage pump chamber 50 b through its lower outlet side into the second gas passage 72 b. Then, the process gas flows upwardly in the second gas passage 72 b and reaches the upper inlet side of the third-stage pump chamber 50 c. Subsequently, the process gas is compressed in and flows through the third-through sixth-stage pump chambers 50 c-50 f. Thereafter, the process gas is discharged from the lower outlet side of the sixth-stage pump chamber 50 f through the outlet port 54 b out of the main pump 14.

According to this embodiment, since the outer barrel 54 of the pump casing 56 is of the double-walled structure having the gas passages 72 a-72 e defined therein, the interiors of the pump chambers 50 a-50 f are reliably thermally insulated from the exteriors thereof by the high-temperature process gas flowing through the gas passages 72 a-72 e, for thereby maintaining the interior of the main pump 14 at a high temperature to prevent a sublimable gas or the like contained in the process gas from being converted into a solid substance and deposited in the main pump 14, i.e., on the inner circumferential surface of the pump casing 56. Particularly, the high-temperature process gas which flows through the gas passages 72 a-72 e from the lower outlet sides of the pump chambers 50 a-50 e to the upper inlet sides of the pump chambers 50 b-50 f in the next stages is effective to heat the pump chambers 50 a-50 f.

In this embodiment, the outer circumferential surface of the outer barrel 54 of the pump casing 56, except the inlet port 54 a and the outlet port 54 b, is surrounded by a thermally insulative jacket 74 which is of a substantially hollow cylindrical shape. The thermally insulative jacket 74 thermally insulates the interiors of the pump chambers 50 a-50 f from the exteriors thereof, thereby keeping the interiors of the pump chambers 50 a-50 f constant in temperature.

Two side panels 80 a, 80 b are disposed respectively on the end walls 64 a, 64 b of the pump casing 56. The rotational shafts 60 are rotatably supported at their outer ends by bearings 84 a, 84 b housed in bearing housings 82 a, 82 b that are mounted respectively on the side panels 80 a, 80 b. Two lubricant housings 88 a, 88 b, for holding a lubricant therein, are disposed on respective outer side surfaces of the side panels 80 a, 80 b. The electric motor 58 has a motor housing coupled to one of the lubricant housings 88 b. The side panels 80 a, 80 b have respective purge gas passages 90 a, 90 b for supplying a purge gas, such as an N₂ gas or the like, to the portions of the rotational shafts 60 in the side panels 80 a, 80 b to prevent the process gas from flowing out of the pump chambers 50 a-50 f into the bearings 84 a, 84 b. The purge gas supplied from the purge gas passages 90 a, 90 b provides non-contact seals between the side panels 80 a, 80 b and the rotational shafts 60 for protecting the rotational shafts 60 against contact-induced wear. The purge gas, which has flowed through the purge gas passage 90 b, also flows into the atmospheric-vented compartment 52.

According to this embodiment, the atmospheric-vented compartment 52, which is held in fluid communication with the outlet port 54 b and vented to the atmosphere, is defined between the end wall 64 b near the sixth-stage (final-stage) pump chamber 50 f wherein the process gas is of the highest pressure and the side panel 80 b which is disposed adjacent to the end wall 64 b. The lubricant housing 88 b which houses therein the bearing housing 82 with the bearings 84 b disposed therein is mounted on the side panel 80 b. Therefore, even when the pressure in the sixth-stage (final-stage) pump chamber 50 f changes, a chamber R which is positioned outside the atmosphere-vented compartment 52 and houses the bearings 84 b therein, i.e., a chamber R which is surrounded by the side panel 80 b and the lubricant housing 88 b, is maintained essentially at the atmospheric pressure at all times. The process gas, which has flowed into the pump chambers 50 a-50 f, is thus reliably prevented from leaking into the chamber R that is positioned outside the atmosphere-vented compartment 52 and houses the bearings 84 b therein. The bearings 84 b are thus protected against the process gas.

The reasons why the bearings 84 b are protected against the process gas will be described below. If there are non-contact seals between the rotational shafts 60 and the side panel 80 b, and the sixth-stage (final-stage) pump chamber 50 f is positioned directly next to the side panel 80 b, or in other words, if the atmosphere-vented compartment 52 is dispensed with, then the pressure in the chamber R which is positioned outside the atmosphere-vented compartment 52 and houses the bearing 84 b therein, i.e., the chamber R which is surrounded by the side panel 80 b and the lubricant housing 88 b, is substantially equal to the average pressure in the sixth-stage (final-stage) pump chamber 50 f. For example, if the pressure on the outlet side of the sixth-stage pump chamber 50 f is the atmospheric pressure of 760 Torr and the pressure on the inlet side thereof is, e.g., 200 Torr, which is lower than the atmospheric pressure, the pressure in the chamber R which is surrounded by the side panel 80 b and the lubricant housing 88 b is of about 480 Torr (=(760+200)/2). When the pressure on the inlet side of the sixth-stage pump chamber 50 f changes, i.e., increases, from 200 Torr to 300 Torr due to an influx of the process gas from the vacuum chamber, the average pressure in the sixth-stage pump chamber 50 f increases to 530 Torr (=(760+300)/2). Therefore, the process gas flows from the sixth-stage pump chamber 50 f into the chamber R through a gap between the side panel 80 b and the rotational shafts 60 until the pressure in the chamber R rises to 530 Torr.

According to this embodiment, the atmospheric-vented compartment 52, which is held in fluid communication with the outlet port 54 b and vented to the atmosphere, is defined between the end wall 64 b near the sixth-stage (final-stage) pump chamber 50 f and the side panel 80 b which is disposed adjacent to the end wall 64 b. Therefore, even when the pressure (average pressure) in the sixth-stage pump chamber 50 f changes, i.e., increases as described above, the pressure in the atmospheric-vented compartment 52 remains unchanged from the atmospheric pressure, and hence the pressure in the chamber R which is positioned outside the atmosphere-vented compartment 52 and houses the bearings 84 b therein, i.e., a chamber R which is surrounded by the side panel 80 b and the lubricant housing 88 b, is not affected by the change in the pressure in the sixth-stage pump chamber 50 f, but is maintained essentially at the atmospheric pressure.

The vacuum pump 10 thus constructed operates by energizing the electric motor 24 of the booster pump 12 and the electric motor 58 of the main pump 14 to actuate the booster pump 12 and the main pump 14 to discharge the process gas, which has been introduced into, e.g., a vacuum chamber, from the vacuum chamber. As the chamber R which is positioned outside the atmosphere-vented compartment 52 and houses the bearings 84 b therein, i.e., a chamber R which is surrounded by the side panel 80 b and the lubricant housing 88 b, is maintained essentially at the atmospheric pressure at all times, the process gas that has been introduced into the main pump 14 is reliably prevented from leaking into the bearings 84 b. Therefore, the bearings 84 b are protected against the process gas.

While the present invention has been described with reference to preferred embodiments, it is understood that the present invention is not limited to the embodiments described above within the scope of the inventive concept as expressed herein. For example, in this embodiment, the present invention is applied to a multistage roots vacuum pump. However, the principles of the present invention are also applicable to vacuum pumps of different types, e.g., a single-stage roots vacuum pump, a claw vacuum pump, a screw vacuum pump, or a vacuum pump including at least two of roots, claw, and screw pump mechanisms on common rotational shafts.

INDUSTRIAL APPLICABILITY

The present invention is applicable to a vacuum pump for use in a process wherein a sublimable gas or a corrosive gas tends to flow into the vacuum pump. 

1. A vacuum pump comprising: a pump casing having an inlet port, an outlet port, and a pump chamber defined therein; a rotational shaft having opposite ends rotatably supported by bearings and extending longitudinally in the pump casing; and a rotor housed in the pump chamber and coupled to the rotational shaft for rotation in unison with the rotational shaft; wherein the pump casing has an atmosphere-vented compartment defined therein which is held in fluid communication with the outlet port of the pump casing and vented to the atmosphere.
 2. A vacuum pump according to claim 1, wherein the pump chamber comprises a plurality of pump chambers held in fluid communication with each other, and the rotor comprises a plurality of rotors disposed respectively in the pump chambers for rotation in unison with the rotational shaft.
 3. A vacuum pump according to claim 1, further comprising: a side panel disposed on an end wall of the pump casing, the rotational shaft extending through the side panel, the side panel having a purge gas passage defined therein for supplying a purge gas to a portion of the rotational shaft which is disposed in the side panel.
 4. A vacuum pump according to claim 2, further comprising: a side panel disposed on an end wall of the pump casing, the rotational shaft extending through the side panel, the side panel having a purge gas passage defined therein for supplying a purge gas to a portion of the rotational shaft which is disposed in the side panel. 